W-CDMA has been standardized by 3GPP (3rd Generation Partnership Project), and services thereof have been sequentially provided. HSDPA (High Speed Downlink Packet Access) which is a faster communication scheme has been also standardized and services thereof are about to be provided.
Evolved Universal Terrestrial Radio Access (hereinafter, “EUTRA”) has been under consideration by 3GPP. OFDM (Orthogonal Frequency Division Multiplexing) has been proposed for EUTRA downlink by 3GPP. DFT (Discrete Fourier Transform)-spread OFDM, which is a single-carrier communication scheme, has been proposed for an EUTRA uplink.
FIG. 15 illustrates an EUTRA uplink-and-downlink channel structure.
An EUTRA downlink includes a DPiCH (Downlink Pilot Channel), a DSCH (Downlink Synchronization Channel), a Downlink Common Control Channel, a PDCCH (Physical Downlink Control Channel) (L1/L2 (Layer 1/Layer 2) Control Channel), and a DL-SCH (Downlink-Shared Channel).
An EUTRA uplink includes a UPiCH (Uplink Pilot Channel), a RACH (Random Access Channel), a UL-SCH (Uplink-Shared Channel), and a PUCCH (Physical Uplink Control Channel) (see Non-Patent Document 1).
FIG. 16 illustrates an example of RACHs and UL-SCHs being allocated to radio resources. In FIG. 16, horizontal and vertical axes denote time and frequency, respectively. FIG. 16 shows a structure of one radio frame which is divided into multiple radio resources. In this case, each radio resource has a region defined by 1.25 MHz in the frequency direction and 1 ms in the time direction. RACHs and UL-SCHs explained in FIG. 15 are allocated to these regions as illustrated. Thus, the minimum unit of a RACH has 1.25 MHz. In FIG. 16, UPiCHs are shatteringly allocated in an UL-SCH region by symbol or subcarrier. Since multiple channels are prepared for RACHs in EUTRA, multiple random accesses are available at the same time. Synchronization between a mobile station device and a base station device is the primary purpose of the use of RACHs. It has also been considered that a few bits of data for requesting a scheduling of a radio resource assignment are transmitted over a RACH to reduce a connection time (see Non-Patent Document 2).
Only a preamble is transmitted over RACH for synchronization. The preamble includes a signature which is a signal pattern indicative of information. From among tens of signatures preliminarily prepared, some signatures are selected to configure a few bits of data. Currently, 6 bits of data are transmitted by signatures in EUTRA. 64 (i.e., 2 to the 6th power) signatures are prepared for 6 bits of data.
A random ID is assigned to 5 bits of 6 bits of signatures. Any of information items concerning a random access reason, a downlink path loss/CQI (Channel Quality Indicator), or the like, is assigned to the remaining 1 bit (see Non-Patent Document 3).
FIG. 17 is a sequence chart illustrating uplink synchronization using RACH. First, a mobile station device selects a signature based on a random ID, a random access reason, a downlink path loss/CQI, or the like, and transmits a preamble including the signature over the RACH (message Ma1). Upon receiving the preamble from the mobile station device, a base station device compares the preamble with a signal pattern preliminarily stored as a preamble to calculate a synchronization timing shift. Then, the base station device performs a scheduling for transmitting an L2/L3 (Layer 2/Layer 3) message, and allocates a C-RNTI (Cell Radio Network Temporary Identifier) to a mobile station device determined to require the C-RNTI based on the random access reason. Then, the base station device transmits a preamble response including synchronization timing shift information, scheduling information, the C-RNTI, and the random ID (message Ma2). The mobile station device extracts the preamble response including the random ID which is transmitted from the base station device, and transmits an L2/L3 message using the scheduled radio resources (message Ma3). Upon receiving the L2/L3 message, the base station device transmits, to the mobile station device, a contention resolution for determining whether or not a collision is occurring between mobile station devices (message Ma4) (see Non-Patent Document 3).
If multiple mobile station devices select the same signature and RACH for random accesses, the random accesses of the mobile station devices collide with one another. A sequence when a collision of random accesses occurs is explained with reference to FIG. 17. If multiple mobile station devices select the same signature and transmit preambles using the same radio resource block having the same time and frequency (i.e., the same RACH), the messages Ma1 collide. If the base station device cannot detect the message Ma1 due to the collision, the base station device cannot transmit a preamble response (message Ma2). Since the mobile station device cannot receive a preamble response (message Ma2) from the base station device, the mobile station device selects a signature and a RACH again after a given time interval, and then performs a random access. On the other hand, if the base station device can detect a preamble (Ma1) in spite of the collision, the base station device calculates a scheduling for an L2/L3 message and a synchronization timing shift, and then transmits a preamble response (message Ma2) to the mobile station devices. However, all the mobile station devices receive the preamble message, and then transmits an L2/L3 message (message Ma3) using the scheduled resource. Consequently, messages Ma3 from the mobile station devices collide. Since the base station device cannot receive the L2/L3 message due to the collision, the base station device cannot transmit a response. Since none of the mobile station devices receives a response to the L2/L3 message, each of the mobile station devices selects a signature again and then performs a random access.
When uplink synchronization between the mobile station device and the base station device is lost (for example, when data has not been received or transmitted for a long period, and the mobile station device is, for a long period, in a DRX (Discontinuous Reception) state for monitoring a downlink resource assignment signal), and when the base station device resumes a downlink data transmission, the mobile station device cannot transmit an ACK/NACK (Acknowledgement/Negative Acknowledgement) which is a reception response for an HARQ (Hybrid Automatic Repeat Request). This is because the uplink synchronization is lost, and therefore a transmission of the ACK/NACK for the HARQ causes an interference with another mobile station device. For this reason, uplink synchronization has to be established using a random access upon a downlink data transmission resuming. However, there is concern that it takes a long time for the downlink data transmission to be resumed if a collision occurs upon the random access. To prevent this, a proposition has been made in which a collision of random accesses upon a downlink data transmission resuming is prevented by, for example, using a signature dedicated to a downlink data transmission resuming.
FIG. 18 is a sequence chart illustrating a method of preventing a collision of random accesses when a downlink data transmission is resumed.
When the base station device decides to resume a downlink data transmission to a mobile station device with which uplink synchronization is lost, the base station device transmits an uplink synchronization request (message Mb1). This uplink synchronization request is transmitted using an L1/L2 (Layer 1/Layer 2) control channel. The uplink synchronization request includes the signature ID number of a random access to be performed by the mobile station device. This is called a dedicated signature. The mobile station device performs a random access (i.e., transmits a preamble) using the dedicated signature included in the received uplink synchronization request (message Mb2). Upon receiving the preamble including the dedicated signature, the base station device detects a synchronization timing shift based on the preamble. Then, the base station device transmits, as a preamble response, a TA (Timing Advance) command indicative of the synchronization timing shift (message Mb3). After the base station transmits the TA command, the base station device transmits an L1/L2 control channel including a downlink resource assignment (message Mb4), and then resumes a downlink data transmission (message Mb5) (see Non-Patent Document 4).    Non-Patent Document 1: R1-050850 “Physical Channel and Multiplexing in Evolved UTRA Uplink”, 3GPP TSG RAN WG1 Meeting #42 London, UK, Aug. 29-Sep. 2, 2005    Non-Patent Document 2: 3GPP TR (Technical Report) 25.814, V7.0.0 (2006-06), Physical layer aspects for evolved Universal Terrestrial Radio Access (UTRA)    Non-Patent Document 3: 3GPP TS (Technical Specification) 36.300, V0.90 (2007-03), Evolved Universal Terrestrial Radio Access (E-UTRA) and evolved Universal Terrestrial Radio Access Network (E-UTRAN), Overall description Stage 2    Non-Patent Document 4: R2-062165 “UL Synchronization”, 3GPP TSG RAN WG2 Meeting #54 Tallinn, 28 Aug.-1 Sep., 2006